Re: Drawing complex scripts with pango.

My only advice is "talk to behdad".

On Mon, Aug 17, 2009 at 3:22 AM, Martin Sevior<msevior@gmail.com> wrote:
>
> Hi everyone,
>                 Bug 12284 nepali text selection oddities,
>
> http://bugzilla.abisource.com/show_bug.cgi?id=12284
>
> See the effect in this screenshot:
>
> http://bugzilla.abisource.com/attachment.cgi?id=5028
>
> Is caused by the same effect that makes characters "dance" around on
> the screen as we select them.
>
> The problem is that pango doesn't like rendering starting from the
> middle of a run characters. For roman text it's left-more position is
> undefined to within one pixel. For complex scripts like Nepalese where
> two or more unicode values are combined to make one glyph pango just
> doesn't draw the string.
>
> The problem is in fp_TextRun::_draw(...)
>
> The problem code  is this:
>
>                // reset the iterator
>                text.setPosition(iIterPos);
>                m_pRenderInfo->m_iOffset = iMyOffset;
>                m_pRenderInfo->m_iLength =
> iSegmentOffset[iSegment+1]-iSegmentOffset[iSegment];
>                m_pRenderInfo->m_xoff = iX;
>                m_pRenderInfo->m_yoff = yTopOfRun;
>                painter.renderChars(*m_pRenderInfo);
>
> and in particular setting m_pRenderInfo->m_iOffset and m_pRenderInfo->m_iLength.
>
> If we replace this with code with and always do one complete draw for
> the run, rather than draw in segments
>
>        pG->setColor(getFGColor());
>        if(iVisDir == UT_BIDI_RTL)
>                iX -= getWidth();
>        m_pRenderInfo->m_iOffset = 0;
>        m_pRenderInfo->m_iLength = getLength();
>        m_pRenderInfo->m_xoff = iX;
>        m_pRenderInfo->m_yoff = yTopOfRun;
>        painter.renderChars(*m_pRenderInfo);
>
> The dancing characters and fixed and we draw correctly. The downside
> is that we always use the some color for the foreground text instead
> of the
> fancy color selection we currently with this code:
>
>                if(bSegmentSelected[iSegment])
>                {
>                        pG->setColor(_getView()->getColorSelForeground());
>                }
>                else
>                {
>                        pG->setColor(getFGColor());
>                }
>
> I'm personally happy to give up the fancy color. I know that uwog
> didn't not like to do this, but that was before this selection bug
> showed up.
>
> What are peoples opinions on how to proceed? My impression is that for
> complex scripts the output of pango is undefined if you attempt to
> draw in the middle of a run.
>
> Cheers,
>
> Martin
>

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